Method of making lithographic aluminum offset printing plates

ABSTRACT

Method for making lithographic aluminum offset printing plates according to the DTR-process by photo-exposing a photosensitive monosheet layer assemblage comprising a hydrophilic aluminum foil, an intermediate layer comprising hydrophobic polymer beads prepared by polymerization of ethylenically unsaturated monomer(s) and having an average diameter not lower than 0.2 μm, and a silver halide emulsion layer, applying an aqueous alkaline solution to the photo-exposed silver halide emulsion layer in the presence of a developing agent and a silver halide solvent to form a silver image and to allow unreduced silver halide or complexes formed thereof to diffuse image-wise from the developed silver halide emulsion layer to said aluminum foil to produce thereon a silver image, and separating said emulsion layer and said intermediate layer from the imaged aluminum foil. The invention also relates to the photosensitive monosheet layer assemblage used for making such printing plates.

This is a division of application Ser. No. 07/783,244 filed Oct. 28,1991, now U.S. Pat. No. 5,273,858.

BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to a method formaking improved lithographic aluminium offset printing plates accordingto the silver complex diffusion transfer reversal process and to thephotosensitive monosheet layer assemblage used for making such printingplates.

2. Description of the Prior Art

The principles of the silver complex diffusion transfer reversalprocess, briefly called DTR-process herein, have been described in e.g.U.S. Pat. No. 2,352,014.

A lithographic printing plate can be made according to the DTR-process.In U.S. Pat. No. 3,511,656 a method has been described for making aprinting plate by photo-exposing a material comprising in the givensequence a silver halide emulsion layer, a silver-receptive stratumcontaining nuclei for precipitation of silver from diffusingwater-soluble silver complexes, and a base sheet e.g. an aluminium foil,and applying an aqueous alkaline solution of a developing agent andsilver halide solvent to the photo-exposed silver halide emulsion layer,reducing the exposed silver halide, allowing the unreduced silver halideor complexes formed thereof to diffuse from the unexposed areas of thesilver halide emulsion layer to the silver-receptive stratum to producefrom the unreduced silver halide or complexes formed thereof inconjunction with the nuclei a visible silver image in thesilver-receptive stratum, said image being oleophilic ink-receptive, andremoving the photo-exposed silver halide emulsion layer from the surfaceof the silver-receptive stratum with warm water. Printing can beachieved by wetting the imaged silver-receptive stratum with aqueousdampening liquid to wet out the non-imaged areas, coating thesilver-receptive stratum with an ink, which wets out the imaged areas,and pressing the inked surface onto copy sheets for the transfer of theink image thereto. It is possible also to dispense with thesilver-receptive stratum containing nuclei so that the oleophilicink-receptive image is formed directly on the base sheet e.g. analuminium foil, the surface of which has been rendered hydrophilicpreviously by brushing, silicating, anodizing, etching, or the like. Bytreatment with a lacquer the oleophilicity of the silver image can beincreased, if desired.

In U.S. Pat. No. 4,772,535 a light-sensitive lithographic printing platematerial has been described, which material comprises a support e.g. ametal support, an optional subbing or antihalation layer or undercoat, anon-light-sensitive silver halide emulsion layer, a light-sensitivesilver halide emulsion layer, and an image-receiving layer containingphysical development nuclei. The material is exposed image-wise throughthe image-receiving layer and developed to form a diffusion transfersilver image in the image-receiving layer (not in the metal support).The imaged element thus obtained is used as such as a printing platewithout separation of the now useless emulsion layers from the layerthat contains the printing image.

According to EP-A 0,278,766 a lithographic printing plate precursor hasbeen proposed, said precursor comprising a grained and anodizedaluminium foil coated with a sol containing nuclei in a gelatin binderand--according to one embodiment--covered with a silver halide emulsionlayer. Extensive experimentation with a said printing plate precursorhas shown unfortunately that satisfactory printing results can only beobtained on the condition that after development of said precursor, theresidual emulsion layer is removed by washing with hot water (50° C.)and that the image plate is treated with a finisher comprising largeamounts (20 g/l) of trypsin. The use of hot water has severaldisadvantages. The cost of hot water is high. Moreover, hot waterdissolves the proteinic binder, usually gelatin, of the emulsion layer,thus causing decomposition of said layer so that a dirty black wastewater comprising silver particles and dissolved silver salts isobtained, which upon cooling may clog filters and draining pipes. As fortrypsin, this is a proteolytic enzyme that should be present in thefinisher to degrade or decompose the proteinic binder that has adsorbedonto the silver grains precipitating on the aluminium foil during imageformation. Substantial amounts of proteinic binder can indeed easilyreach the silver grains owing to the fact that a silver-receptivestratum comprising gelatin and a gelatin silver halide emulsion layerhave been coated directly on the aluminium foil. After oleophilizationof said silver image the adsorbed gelatin, which is inherentlyhydrophilic, constitutes an undesired hydrophilic element in the masterimage so that prints having an insatisfactory quality are obtained.Moreover, said trypsin, which is essential to degrade the proteinicbinder in the silver image, is extraordinarily expensive and isecologically harmful as can be derived from i.a. Sigma Aldrich Libraryof Chemical Safety Data: MSD Book, 2,35553A,B,C and from Registry ofToxic Effects of Chemical Substances, YN507500.

In addition to the above disadvantages it has also been established thatthe gelatin present in substantial amounts in the nuclei-containinglayer and in the emulsion of the lithographic printing plate precursorlayer has a corrosive effect on the aluminium foil. The corrosive effectof gelatin on aluminium has indeed been described by J. H. Penn and G.A. W. Murray in Br. Corros. J., 1967, Vol .2, September, pages 193-4.Even though the corrosive influence of gelatin on the aluminium foil maybe limited thanks to the presence of the anodization layer thereon, thisprotection is incomplete owing to random defects in the continuity ofthe anodization layer.

Furthermore, it is generally known that aluminium ions have a hardeninginfluence on gelatin (see e.g. the paragraph bridging pages 78 and 79 of"The Theory of the Photographic Process" 4th Ed., edited by T. H.James). Aluminium ions of the foil can indeed cause a hardening reactionin the gelatin layers so that removal of the emulsion layer graduallybecomes more difficult.

Finally, as a result of the corrosive effect of gelatin on aluminium andthe hardening reaction caused by aluminium in the gelatin layers, theshelf-life of the lithographic printing plate precursor is limitedsubstantially.

According to U.S. Ser. No. 07/552,945 these disadvantages have beencircumvented for the major part by providing between the aluminium foiland the silver halide emulsion layer a thin water-swellable intermediatelayer comprising for at least 70% of its total weight at least onenon-proteinic hydrophilic film-forming polymer.

However, since on a microscopic scale the grained surface of analuminium foil is very rugged with deep valleys and steep peaks, it ispractically impossible to completely cover this rugged aluminium surfacewith just a thin water-swellable intermediate layer. It was, therefore,tried to fill up this ruggedness by enhancing the thickness of thewater-swellable intermediate layer, but although a thicker intermediatelayer offered a solution with respect to creating a more efficientbarrier between the aluminium surface and the emulsion layer, it led toanother disadvantage. During diffusion transfer the silver saltsmigrating from the emulsion layer to the aluminium surface through thethick water-swollen intermediate layer have to cover a longer diffusionpath so that lateral diffusion becomes more substantial. As aconsequence, the deposition of silver on the aluminium surface and thesharpness of the transferred silver image are reduced.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved method for making lithographic aluminium printing platesaccording to the DTR-process in a convenient and ecologically as well aseconomically acceptable way.

It is another object of the present invention to provide aphotosensitive monosheet layer assemblage for making a lithographicaluminium printing plate, by which sharp high quality prints can be madeaccording to the DTR-process, said photosensitive monosheet layerassemblage having an improved shelflife.

These and other objects are achieved by providing an improved method formaking lithographic aluminium offset printing plates according to theDTR-process comprising the steps of:

(a) photo-exposing a photosensitive monosheet layer assemblagecomprising:

--a hydrophilic grained and anodized aluminium foil,

--an intermediate layer comprising hydrophobic polymer beads prepared bypolymerization of at least one ethylenically unsaturated monomer andhaving an average diameter not lower than 0.2 μm, and

at least one silver halide emulsion layer,

(b) applying an aqueous alkaline solution to the photo-exposed silverhalide emulsion layer in the presence of at least one developing agentand at least one silver halide solvent to form a silver image and toallow unreduced silver halide or complexes formed thereof to diffuseimage-wise from the developed silver halide emulsion layer to saidhydrophilic grained and anodized aluminium foil to produce thereon asilver image, and

(c) separating said at least one emulsion layer and said intermediatelayer from the imaged hydrophilic grained and anodized aluminium foil.

In the above step (c) said separating can be accomplished e.g. by thesteps of:

--bringing said monosheet layer assemblage with its side showing said atleast one emulsion layer during the period of time starting with theapplication of said aqueous alkaline solution and ending with saidformation of a silver image on said hydrophilic grained and anodizedsurface in contact with a receiving means, said at least one emulsionlayer and said intermediate layer being wet with said aqueous alkalinesolution having an adherence to said receiving means that is strongerthan that to the imaged hydrophilic grained and anodized aluminium foiland

--peeling off said at least one emulsion layer and said intermediatelayer adhering to said receiving means from the imaged hydrophilicgrained and anodized aluminium foil,

or by the steps of:

--removing said monosheet layer assemblage from the alkalinesolution-applying means after completion of said formation of a silverimage on said hydrophilic grained and anodized aluminium foil and

--detaching said at least one emulsion layer and said intermediate layerfrom the imaged hydrophilic grained and anodized aluminium foil with theaid of unheated water or unheated aqueous medium,

or by the steps of:

--removing said monosheet layer assemblage from the alkalinesolution-applying means after completion of said formation of a silverimage on said hydrophilic grained and anodized aluminium foil and

--detaching said at least one emulsion layer and said intermediatelayer, while still being wet with alkaline solution or while being wetwith unheated water or an unheated aqueous medium applied theretosubsequent to the removal of said monosheet layer assemblage from saidalkaline solution, from the imaged hydrophilic grained and anodizedaluminium foil with the aid of an air current directed onto an edge ofsaid monosheet layer assemblage.

The imaged hydrophilic grained and anodized aluminium foil can be usedfor making prints as follows:

--treating said imaged aluminium foil by rubbing with a fixer to enhancethe water-receptivity of the non-image areas and to make the image areasoleophilic ink-receptive,

--wetting said imaged aluminium foil with an aqueous dampening liquid towet out the non-imaged areas,

--coating said imaged aluminium foil with an ink that wets out theimaged areas, and

--pressing the inked surface of the resulting lithographic aluminiumoffset printing plate in an offset press onto a blanket that transfersthe ink onto copy sheets.

The present invention also provides a photosensitive monosheet layerassemblage for making a lithographic aluminium printing plate accordingto the DTR-process, said assemblage comprising in the given sequence ahydrophilic grained and anodized aluminium foil and at least one silverhalide emulsion layer, wherein an intermediate layer comprisinghydrophobic polymer beads prepared by polymerization of at least oneethylenically unsaturated monomer and having an average diameter notlower than 0.2 μm is provided between said hydrophilic grained andanodized aluminium foil and said at least one silver halide emulsionlayer.

The photosensitive monosheet layer assemblage according to the presentinvention can be made by coating a hydrophilic grained and anodizedsurface of an aluminium foil with an intermediate layer comprisinghydrophobic polymer beads prepared by polymerization of at least oneethylenically unsaturated monomer and having an average diameter notlower than 0.2 μm, and coating said intermediate layer with at least onesilver halide emulsion layer.

According to another embodiment the said photosensitive monosheet layerassemblage can be prepared by the steps of:

--coating a temporary base with at least one silver halide emulsionlayer,

--coating said at least one silver halide emulsion layer with anintermediate layer comprising hydrophobic polymer beads prepared bypolymerization of at least one ethylenically unsaturated monomer andhaving an average diameter not lower than 0.2 μm, and

--pressing the thus formed photosensitive layer packet with its sidecarrying said intermediate layer against the hydrophilic grained andanodized surface of an aluminium foil, which has been wet with anaqueous moistening liquid, the said temporary base being removed beforeor after said photo-exposure.

The invention thus also provides a photosensitive layer packet intendedfor making a lithographic aluminium printing plate according to theDTR-process, wherein said photosensitive layer packet comprises atemporary base temporarily carrying a separable layer sandwichcomprising in the given sequence at least one silver halide emulsionlayer and an intermediate layer comprising hydrophobic polymer beadsprepared by polymerization of at least one ethylenically unsaturatedmonomer and having an average diameter not lower than 0.2 μm, said layersandwich being transferable onto a wet separate hydrophilic grained andanodized aluminium foil when pressed thereon. The temporary base can bepeeled off before or after photo-exposure of the silver halide emulsionlayer(s).

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention for making prints according to theDTR-process by means of an improved lithographic aluminium offsetprinting plate comprises the consecutive steps of:

(1) making a photosensitive monosheet layer assemblage e.g. by the stepsof:

--coating the hydrophilic grained and anodized surface of an aluminiumfoil, preferably after having applied thereto a silver-receptive stratumcontaining development nuclei for precipitation of silver from diffusingwater-soluble silver complexes, with an intermediate layer comprisinghydrophobic polymer beads having an average diameter not lower than 0.2μm and having been prepared by polymerization of at least oneethylenically unsaturated monomer e.g. chosen from the group consistingof alkyl methacrylates e.g. methyl methacrylate, ethyl methacrylate,propyl methacrylate, butyl methacrylate and the higher methacrylatessuch as stearyl methacrylate, substituted alkyl methacrylates e.g.hydroxyethyl methacrylate, alkyl acrylates, substituted alkyl acrylates,styrene, substituted styrene e.g. chlorostyrene, vinyltoluene andsubstituted vinyltoluene e.g. vinylbenzyl chloride and the homologuesthereof, butadiene, substituted butadiene e.g. chlorobutadiene,2-methylbutadiene, isobutylene, and substituted isobutylene, andvinylpyridine e.g. 2- and 4-vinyl-pyridine, said intermediate layerpreferably comprising an antihalation dye or pigment, and

--coating said intermediate layer with at least one silver halideemulsion layer, at least one silver halide emulsion layer beingphotosensitive and optionally comprising a light-screening dye,

or by the steps of:

--coating a temporary base, preferably a cellulose triacetate orpolyethylene terephtalate film base, with at least one silver halideemulsion layer, at least one silver halide emulsion layer beingphotosensitive and optionally comprising a light-screening dye,

--coating said at least one silver halide emulsion layer with anintermediate layer comprising hydrophobic polymer beads having anaverage diameter not lower than 0.2 μm and having been prepared bypolymerization of at least one ethylenically unsaturated monomer, saidintermediate layer optionally comprising an antihalation dye or pigment,

--pressing the thus formed photosensitive layer packet with its sidecarrying said intermediate layer against the hydrophilic grained andanodized surface of an aluminium foil, which has been wet with anaqueous moistening liquid that may comprise additives,

--removing said temporary base to leave a photosensitive monosheet layerassemblage supported by said aluminium foil and optionally drying saidphotosensitive monosheet layer assemblage supported by said aluminiumfoil,

(2)--photo-exposing the thus formed photosensitive monosheet layerassemblage,

(3)--applying an aqueous alkaline solution to the photo-exposed silverhalide emulsion layer in the presence of at least one developing agentand at least one silver halide solvent to form a silver image and toallow unreduced silver halide or complexes formed thereof to diffuseimage-wise from the developed silver halide emulsion layer to saidhydrophilic grained and anodized surface to produce thereon a silverimage preferably under the catalytic influence of development nuclei,

(4)--separating said at least one emulsion layer and said intermediatelayer from the imaged hydrophilic grained and anodized surface, saidseparating being accomplished e.g. by the steps of:

--bringing said monosheet layer assemblage with its side showing said atleast one emulsion layer during the period of time that starts with theapplication of said aqueous alkaline solution and ends with saidformation of a silver image on said hydrophilic grained and anodizedsurface in contact with a receiving means, said at least one emulsionlayer and said intermediate layer being wet with said aqueous alkalinesolution having an adherence to said receiving means that is strongerthan that to the imaged hydrophilic grained and anodized surface and

--peeling off said at least one emulsion layer and said intermediatelayer adhering to said receiving means from the imaged hydrophilicgrained and anodized surface,

or by the steps of:

--removing said monosheet layer assemblage from the alkalinesolution-applying means e.g. a bath or a roller system such as a lickroller, after completion of said formation of a silver image on saidhydrophilic grained and anodized surface and

--detaching said at least one emulsion layer and said intermediate layerfrom the imaged hydrophilic grained and anodized surface with the aid ofunheated water or unheated aqueous medium, said detaching beingperformed e.g.:

--according to a preferred mode, by rinsing with a spray or jet ofunheated water or unheated aqueous medium directed onto said at leastone emulsion layer and said intermediate layer, or

--by agitating or shaking a tray comprising said monosheet layerassemblage immersed in unheated water or unheated aqueous medium, oragitating said monosheet layer assemblage while being immersed inunheated water or unheated aqueous medium,

--according to another preferred mode, by pressing said monosheet layerassemblage with its side showing said at least one emulsion layer, whilebeing moistened with unheated water or unheated aqueous medium, againsta receiving sheet such as a polyethylene-coated paper sheet and peelingoff said receiving sheet together with said at least one emulsion layerand said intermediate layer, which remain strongly adhering to saidreceiving sheet, from said imaged aluminium foil,

the mechanical effect obtained by said rinsing or agitating or pressingagainst a receiving sheet and peeling off, being sufficient to eliminatesaid at least one emulsion layer and said intermediate layer from theimaged hydrophilic grained and anodized surface,

or by the preferential steps of:

--removing the monosheet layer assemblage from the alkalinesolution-applying means e.g. a bath or a roller system such as a lickroller, after completion of said formation of a silver image on saidhydrophilic grained and anodized surface and

--detaching said at least one emulsion layer and said intermediatelayer, while still being wet with alkaline solution or while being wetwith unheated water or an unheated aqueous medium applied theretosubsequent to the removal of said monosheet layer assemblage from saidalkaline solution, from the imaged hydrophilic grained and anodizedaluminium foil with the aid of an air current directed onto an edge ofsaid monosheet layer assemblage,

the mechanical effect obtained by said air current being sufficient toeliminate said at least one emulsion layer and said intermediate layerfrom the imaged hydrophilic grained and anodized surface,

--treating said imaged surface by rubbing it with a fixer to enhance thewater-receptivity of the non-image areas and to make the image areasoleophilic ink-receptive,

(6)--wetting said imaged surface with an aqueous dampening liquid to wetout the non-imaged areas,

(7)--coating said imaged surface with an ink that wets out the imagedareas, and

(8)--pressing the inked surface of said resulting lithographic aluminiumoffset printing plate in an offset press onto a blanket that transfersthe ink onto copy sheets.

An above-described embodiment of the method of the invention comprisesthe following consecutive steps:

--making a photosensitive monosheet layer assemblage by coating atemporary base successively with at least one silver halide emulsionlayer and a said intermediate layer,

--next pressing the thus formed photosensitive layer packet against analuminium foil, which has been wet with an aqueous moistening liquid, totransfer said intermediate layer and said emulsion layer onto said wetfoil,

--removing said temporary base to leave a photosensitive monosheet layerassemblage supported by said aluminium foil,

--photo-exposing the thus formed photosensitive monosheet layerassemblage.

It has, however, been found that this sequence of steps may be alteredin the sense that said temporary base is not removed before thephoto-exposure step. Thus, according to this interesting variant saidphotosensitive layer packet is pressed against a wet aluminium foil totransfer said intermediate layer and said emulsion layer onto said wetfoil, the aluminium side of the resulting sandwich is dried slightly,and the photo-exposure of the sandwich is then performed through saidtemporary base, which for this embodiment obviously is a transparentfilm base. It is further possible to expose the photosensitive silverhalide emulsion layer(s) on the temporary base, then to press the packetagainst the wet aluminium foil, and finally to remove the temporarybase. Thanks to the presence of the temporary base the silver halideemulsion layer(s) find themselves protected from mechanical deformation,especially in wet condition.

The invention also provides a photosensitive monosheet layer assemblagefor making a lithographic aluminium printing plate for use according tothe DTR-process, said assemblage comprising in the given sequence:

--a hydrophilic grained and anodized aluminium foil, the anodizationlayer of which may be coloured with an antihalation dye or pigment,

--optionally a silver-receptive stratum containing development nucleifor precipitation of silver from diffusing water-soluble silvercomplexes, which stratum may comprise an antihalation dye or pigment,

--at least one silver halide emulsion layer, at least part of the silverhalide emulsion being photosensitive and optionally comprising alight-screening dye,

wherein an intermediate layer is provided between on the one hand saidsilver-receptive stratum or in the absence of said silver-receptivestratum said hydrophilic grained and anodized surface and on the otherhand said at least one silver halide emulsion layer, said intermediatelayer comprising hydrophobic polymer beads having an average diameternot lower than 0.2 μm and having been prepared by polymerization of atleast one ethylenically unsaturated monomer, said intermediate layeroptionally comprising an antihalation dye or pigment and optionallycomprising a matting or spacing agent. Preferably, said intermediatelayer in dry condition comprises said hydrophobic polymer beads in anamount of up to 80% of its total weight.

The invention further provides a photosensitive layer packet and aseparate hydrophilic grained and anodized aluminium foil, togetherintended for use in transferring a transferable layer sandwich of saidphotosensitive layer packet by separation from a temporary base ontosaid separate hydrophilic grained and anodized aluminium foil in wetcondition to form a photosensitive monosheet layer assemblage therewithfor making a lithographic aluminium printing plate for use according tothe DTR-process. Said photosensitive layer packet comprises a temporarybase temporarily carrying a said separable layer sandwich comprising inthe given sequence at least one silver halide emulsion layer, at leastpart of the silver halide emulsion being photosensitive and optionallycomprising a light-screening dye, and an intermediate layer comprisinghydrophobic polymer beads prepared by polymerization of at least oneethylenically unsaturated monomer and having an average diameter notlower than 0.2 μm, said intermediate layer optionally comprising anantihalation dye or pigment and optionally comprising a matting orspacing agent, and said layer sandwich being transferable onto said wetseparate aluminium foil when pressed thereon. Preferably, saidintermediate layer ir dry condition comprises said hydrophobic polymerbeads in an amount of up to 80% of its total weight.

As referred to hereinbefore the removal of the temporary base can bedelayed until the photo-exposed material is to be treated with theaqueous alkaline solution.

It has been established that thanks to the presence of the thin layercomprising hydrophobic polymer beads prepared by polymerization of atleast one ethylenically unsaturated monomer and having an averagediameter not lower than 0.2 μm as an intermediate layer between on theone hand said silver-receptive stratum or in the absence of saidsilver-receptive stratum said hydrophilic grained and anodized surfaceand on the other hand said at least one silver halide emulsion layer, anefficient barrier is formed against the mutual above-mentioned adverseeffects that a proteinic binder, usually gelatin, and aluminium exert oneach other when in contact with one another. By eliminating or reducingthese adverse effects in this way the shelf-life of the photosensitivemonosheet layer assemblage is increased considerably. It is self-evidentthat when the above-defined photosensitive layer packet and separatealuminium foil are used, the contact between the proteinic binder andaluminium is excluded during storage of the separate materials so thatthe above-mentioned adverse effects cannot occur. Thanks to theelimination or reduction of these adverse effects the printing platesmade according to any of the embodiments of the method of the presentinvention yield prints having a high quality. Moreover, no ecologicallyharmful substances such as enzymes e.g. trypsin have to be incorporatedinto the finisher liquids such as e.g. the fixer, which are needed toprepare the printing plate. Furthermore, the process is convenient andeconomically interesting in that no hot water is needed. Since therinsing water or aqueous medium may be in unheated condition, it doesnot dissolve the proteinic binder of the developed emulsion layer. Infact the emulsion layer detaches in the form of swollen flakes that caneasily be filtered from the rinsing water or aqueous medium. No cloggingof filters and draining pipes can be occasioned. After filtration therinsing water or aqueous medium is clear and non-pollutant and comprisessubstantially no silver particles nor silver salts so that it may bedischarged in the sewage. By the term "unheated" as used herein inconnection with the rinsing water or aqueous medium is meant that noexternal heating means are applied for heating said water or aqueousmedium. However, it is self-evident that the invention also encompassesthe use of warm water.

Due to the thinness of the intermediate layer lateral diffusion thereinof the migrating silver salts during diffusion transfer is neglegible sothat sharp transfer images are obtained on the aluminium surface. As aresult, sharp high quality prints can be made with the aluminiumprinting plates.

The mechanical effect either obtained by said rinsing with a spray orjet directed onto said at least one emulsion layer and said intermediatelayer or obtained by said agitation or shaking or obtained by saidpressure against said receiving sheet must be sufficient to detach theselayers from said imaged hydrophilic grained and anodized surface.

The hydrophobic polymer beads for use in the intermediate layer of thepresent invention are prepared by polymerization of at least oneethylenically unsaturated monomer. Preferred polymer beads are e.g.polymethyl methacrylate beads, polystyrene beads, ethyl acrylate/stearylmethacrylate copolymer beads, methacrylic acid/methylmethacrylate/stearyl methacrylate copolymer beads, and beads prepared asdescribed in U.S. Pat. Nos. 4,614,708 and 4,861,818.

The average size of the hydrophobic polymer beads for use in accordancewith the invention is, of course, determined by the nature of said atleast one ethylenically unsaturated monomer, but can also be controlledby adjustment of other reaction parameters as described in theabove-mentioned U.S. Pat. No. 4,614,708 and 4,861,818.

The preparation of hydrophobic polymer beads for use in accordance withthe invention is illustrated by the following preparation example.

Preparation example: polymethyl methacrylate beads

At room temperature 271.73 g of a 20% by weight solution ofco(styrene/maleic acid monosodium salt (pH=7) and 3752.2 g ofdemineralized water are mixed in a 10 1 cylindrical double-walledreaction vessel. The solution is stirred by means of a rotor having alength of 15.5 cm and a width of 4 cm set at a speed of 100 rpm.

The reaction vessel is equipped with a reflux condenser and a nitrogeninlet reaching below the liquid level and is sealed. Hot water (65° C.)is fed through the double wall of the reaction vessel so that after 1 hthe temperature of the solution reaches 65° C.

A continuous inlet of nitrogen keeps the solution free from oxygen.

An amount of 10.86 g of potassium persulphate is then added at once tothe solution. Heating of the solution to 65° C. and stirring arecontinued.

After this preliminary reaction step the stirring speed is maintained at100 rpm. Next, 21.74 g of ARKOPAL N60 commercially available fromHoechst, 2137.48 ml of methanol, and 1086.95 g of methyl methacrylate(not distilled preliminarily) are added in the given sequenve undernitrogen atmosphere.

At this very moment the following parameters should be met:

--1.0 g of potassium persulphate is present per 100 g of methylmethacrylate

--2.0 g of ARKOPAL N60 is present per 100 g of methyl methacrylate

--the ratio by volume of methanol/water is 35/65

--the concentration of monomer at the start of the reaction is 1.50 molof methyl methacrylate per litre

--the stirring speed is 100 rpm.

The temperature of the water-bath is continuously kept at 65° C. Thepolymerization reaction is very slightly exothermic so that thetemperature in the reaction vessel rises to a maximum of 65.5° C. Atthis moment a weak flow of cold tap water is pumped in addition to thehot water (65° C.) into the double wall, the flow of cold water beingadjusted automatically with the aid of a contact thermometer, a relay,and an automatic water valve in such a way that as soon as thetemperature in the reaction vessel drops to 65° C. the flow of coldwater is interrupted immediately.

At the start of the polymerization reaction the solution has a clearaspect, but after some 30 min the solution becomes turbid and thenslowly and gradually turns into a milky white dispersion.

Eventually, after a total polymerization period of 18 h the supply ofhot water and of nitrogen is stopped. The bead dispersion obtained iscooled by means of cold tap water to about 30° C. with continuousstirring and then filtered through a nylon cloth having a mesh width of60×60 μm. Filtering is easy, a maximum of 2.0 g of polymer in amorphousstate remaining on the cloth.

Yield: 6795 g of dispersion having 16.5 g of dry residue per 100 g ofdispersion (pH=5.6). The polymethyl methacrylate beads obtained have anaverage diameter of 1 μm.

The presence of antihalation dyes or pigments in the intermediate layeroffers the supplemental advantage that said layer loosens more easilyfrom the imaged aluminium foil. To further improve the loosening of theintermediate layer, said layer may also comprise matting agents orspacing agents e.g. finely divided silica particles. In case thephotosensitive layer assemblage consisting of a temporary base,photosensitive silver halide emulsion layer(s), and an intermediatelayer is to be exposed in a vacuum contact exposure unit such matting orspacing agents also promote an effective vacuum suction.

According to a special embodiment of the present invention theintermediate layer may in addition to the hydrophobic polymer beadsprepared by polymerization of at least one ethylenically unsaturatedmonomer also comprise an aqueous dispersion of alkali-solublehydrophobic polymer particles, all particles having an average particlesize not lower than 0.2 μm. The amount of said aqueous dispersion ofhydrophobic polymer particles present in said intermediate layer may beup to 10% by weight calculated on the total weight of said layer. Theuse of such alkali-soluble hydrophobic polymer particles in addition tothe hydrophobic polymer beads of the present invention in theintermediate layer offers the advantage that during processing of thephoto-exposed emulsion layer(s) in an aqueous alkaline solution at leastpart of said alkali-soluble hydrophobic polymer particles are dissolvedso that cavities or holes are formed in the intermediate layer, throughwhich the diffusion transfer of migrating silver salts to the aluminiumsurface is facilitated. As a result, a high density buildup oftransferred silver is accomplished.

Non-proteinic hydrophilic film-forming polymers may be present in theintermediate layer in an amount of e.g. up to 20% by weight of the totalweight of said layer. Suitable non-proteinic hydrophilic film-formingpolymers are e.g. polyvinyl alcohol, polyvinyl pyrrolidone, polyethyleneoxide, partly hydrolyzed polyvinyl acetate, sulphonated polystyrene,hydroxyethyl cellulose, carboxymethyl cellulose, cellulose acetatehydrogen phthalate, dextran, dextrins or derivatives thereof, starch,gum arabic, and alginic acid derivatives such as salts or estersthereof. It is also possible to use mixtures of two or more differentnon-proteinic hydrophilic film-forming polymers.

According to the preferred way of separating said at least one emulsionlayer and said intermediate layer from the imaged aluminium foil bymeans of an airstream, the process of the present invention is even moreconvenient and economically interesting in that the blown off layersdetach as a cohesive mass that in contrast with the other separationmethods of the present invention does not need any further separation orfiltration from water or aqueous medium. The loosened cohesive massdries rapidly and recovery of silver therefrom is easy and complete.

According to this preferred way of separating, the emulsion layer andintermediate layer, while both still being wet with said aqueousalkaline solution or while being moistened with unheated water or anunheated aqueous medium applied thereto subsequent to the removal ofsaid monosheet layer assemblage from said alkaline solution, aredetached from the imaged aluminium foil by means of an airstream e.g. ajet of compressed air, preferably an airstream blown from a slot orificee.g. an air knife or air doctor, said airstream being directed onto thelengthwise or breadthwise edge of said monosheet layer assemblage. Thedevice capable of generating the airstream used for separating theemulsion layer and intermediate layer from the imaged aluminium foile.g. an air knife can be incorporated as a separate station in theprocessing apparatus. In that case the latter processing apparatusincludes a station for applying aqueous alkaline solution to thephoto-exposed emulsion layer and allowing unreduced silver halide orcomplexes formed thereof to diffuse to the aluminium foil and a saidstation for separating by means of an airstream. As mentioned beforeunheated water or an unheated aqueous medium may be applied optionallyto the monosheet layer assemblage after removal thereof from saidalkaline solution and prior to said detaching by means of an airstream.It is self-evident that the invention also encompasses the use of hotwater.

According to the variant using said receiving sheet, the receiving sheettogether with said at least one emulsion layer and said intermediatelayer, which remain strongly adhering to said receiving sheet, arepeeled off from said imaged aluminium foil.

The restricted softening and swelling in alkaline activating ordeveloping solutions is due to ions such as e.g. sulphite andthiosulphate ions, which are present conventionally therein. Thisrestricted swelling is comparable to that observed for emulsion gellayers. The typical changes in the degree of swell of an emulsion gellayer as it passes through the development in an alkaline solutioncausing a limited swell and subsequently through the rinsing with awater bath causing high swell have been shown in FIG. 15.17 on page 453of "The Theory of the Photographic Process" 4th edition, edited by T. H.James, Macmillan Publishing Co., Inc. New York.

The aqueous moistening liquid used to wet said aluminium foil so thatthe transferable layers can be transferred thereto may consist of wateror may be an aqueous solution comprising additives such as i.a.surface-active agents, a water-miscible alcohol e.g. ethanol, andhardeners including latent hardeners.

On the one hand the nature of the hydrophobic polymer beads prepared bypolymerization of at least one ethylenically unsaturated monomer andhaving an average diameter not lower than 0.2 μm is chosen such thatwhen a layer thereof--even a layer thereof comprising up to 20% byweight of any non-proteinic hydrophilic film-forming polymer--isimmersed in said aqueous alkaline solution, whether it is an activatingsolution or a developing solution. softening and swelling of said layeris practically inexistent so that it remains adhering to the aluminiumfoil. On the other hand the nature of the hydrophobic polymer beads andof any non-proteinic hydrophilic film-forming polymer used in admixturetherewith is chosen such that when:

--either a spray or jet of water or of an aqueous non-alkaline medium isdirected onto said at least one emulsion layer and said intermediatelayer,

--or a tray comprising said monosheet layer assemblage immersed inunheated water or unheated aqueous medium is agitated or shaken, or saidmonosheet layer assemblage is agitated while being immersed in unheatedwater or unheated aqueous medium,

--or said monosheet layer assemblage is pressed with its side showingsaid at least one emulsion layer, while being moistened with unheatedwater or unheated aqueous medium, against a receiving sheet such as e.g.a subbed cellulose triacetate film sheet,

the intermediate layer is caused to detach from the aluminium foil andcarry along the swollen emulsion layer(s).

For use according to the preferred method of separating the emulsionlayer(s) and intermediate layer from the aluminium foil by means of anairstream, the non-proteinic hydrophilic film-forming polymer or mixtureof non-proteinic hydrophilic polymers optionally present in saidintermediate layer is such that when said intermediate layer is immersedin said aqueous alkaline solution, whether it is an activating solutionor a developing solution, or is immersed, after having been removed fromsaid aqueous alkaline solution, in unheated water or in an unheatedaqueous medium, softening and swelling of said polymer(s) is poor and isrestricted substantially to an extent that the layer containing saidpolymer(s) together with said hydrophobic polymer beads remains adheringto the aluminium foil. On the other hand the nature of the non-proteinichydrophilic film-forming polymer or mixture of non-proteinic hydrophilicpolymers is such that when an airstream is directed onto an edge of saidmonosheet layer assemblage, said layers both still being wet withabsorbed aqueous alkaline solution or with subsequently applied unheatedwater or unheated aqueous medium, the intermediate layer detachesreadily from said aluminium foil while carrying along the swollenemulsion layer(s).

The thickness of the intermediate layer may vary between wide limits.However, the coating weight of the composition for making theintermediate layer is preferably not lower than 0.1 g/m2, sinceotherwise an inadequate or insufficient barrier against theabove-mentioned adverse effects may be created. On the other hand thecoating weight of the composition for making the intermediate layerpreferably must not be higher than 5.0 g/m2, since owing to an increasedthickness of the layer the diffusion path of the migrating silvercomplexes would be extended such that the chances of lateral diffusionof these migrating silver complexes would increase. Lateral diffusionmay, of course, lead to insufficient sharpness of the silver imageformed on the aluminium foil. Preferably, the composition for making theintermediate layer is thus coated at a ratio of from about 0.1 to about5 g/m2.

The intermediate layer may incorporate at least one dye or pigment forantihalation purposes. The usual dyes and pigments can be chosendepending upon the desired absorption spectrum of the resulting layerrelative to the spectral sensitivity of the silver halide emulsionlayer(s).

The hydrophobic polymer beads of the intermediate layer can be loadedwith a hydrophobic dye for antihalation purposes according to theso-called technique of loading latices with a hydrophobic substance asdescribed in i.a. DE-A 2,541,274 and 2,541,230.

According to a preferred embodiment of the present invention thehydrophobic polymer beads of the intermediate layer can be loaded with adye for antihalation purposes according to the method described in theEP Patent Application 483,416, filed on Nov. 2, 1990 and entitled"Method of making aqueous loaded latex compositions".

The intermediate layer may in addition to dyes or pigments and mattingagents or spacing agents comprise further ingredients such as i.a.developing agents, other development ingredients, base precursors,silver halide solvents, and anticorrosive agents.

The aluminium foil of the photosensitive monosheet layer assemblage ofthe present invention can be made of pure aluminium or of an aluminiumalloy, the aluminium content of which is at least 95%. A useful alloy ise.g. one comprising 99.55% by weight of Al, 0.29% of Fe, 0.10% of Si,0.004% of Cu, 0.002% of Mn, 0.02% of Ti, and 0.03% of Zn. The thicknessof the foil usually ranges from about 0.13 to about 0.50 mm.

The preparation of aluminium or aluminium alloy foils for lithographicoffset printing comprises the following steps: graining, anodizing, andoptionally sealing of the foil.

Graining and anodization of the foil are necessary to obtain alithographic printing plate that allows to produce high-quality printsin accordance with the present invention. Sealing is not necessary butmay still improve the printing results.

Graining of the aluminium surface can be carried out mechanically orelectrolytically in any known way. The roughness produced by thegraining is measured as a centre line average value expressed in μm andpreferably varies from about 0.2 to about 1.5 μm.

The anodization of the aluminium foil can be performed in electrolytessuch as e.g. chromic acid, oxalic acid, sodium carbonate, sodiumhydroxide, and mixtures thereof. Preferably, the anodization of thealuminium is performed in dilute aqueous sulphuric acid medium until thedesired thickness of the anodization layer is reached. The aluminiumfoil may be anodized on both sides. The thickness of the anodizationlayer is most accurately measured by making a micrographic cut but canbe determined likewise by dissolving the anodized layer and weighing theplate before dissolution treatment and subsequent thereto. Good resultsare obtained with an anodization layer thickness of about 0.4 to about2.00 μm. To promote the image sharpness and, as a consequence thereof,the sharpness of the final printed copy, the anodization layer may becoloured in the mass with an antihalation dye or pigment e.g. asdescribed in JA-A 58-14797. The dye or pigment or a combination of dyesor pigments used for such colouring in the mass are chosen such thatthey prevent or reduce halation in silver halide emulsions having anydesired photosensitivity range comprised between 300 and 900 nm.

After the anodizing step the anodic surface may be sealed. Sealing ofthe pores of the aluminium oxide layer formed by anodization is atechnique known to those skilled in the art of aluminium anodization.This technique has been described in e.g. the "Belgisch-Nederlandstijdschrift voor Oppervlaktetechnieken van materialen", 24stejaargang/januari 1980, under the title "Sealing-kwaliteit ensealing-controle van geanodiseerd Aluminium". Different types of sealingof the porous anodized aluminium surface exist. An advantageous sealingmethod is the hydration-sealing method, according to which the pores areclosed or partially closed through water-acceptance so that hydratedneedle-like aluminium oxide crystals (bohmite) are formed. The anodicsurface of the aluminium foil can thus be rinsed with water at 70°-100°C. or with steam. The hot sealing water may comprise additives such asnickel salts to improve the sealing effect. The sealing can also beperformed by treatment of the anodic surface with an aqueous solutioncomprising phosphate ions or silicates. Thanks to the sealing treatmentthe anodic layer is rendered substantially non-porous so that longerpress runs can be made with the printing plate obtained. As a result ofthe sealing the occurrence of fog in the non-printing areas of theprinting plate is avoided substantially.

The graining, anodizing, and sealing of the aluminium foil can beperformed as described in e.g. U.S. Pat. No. 3,861,917 and in thedocuments referred to therein.

To promote the image sharpness and, as a consequence thereof, thesharpness of the final printed copy, the grained, anodized, andoptionally sealed aluminium foil can be provided with a very thinantihalation coating of a dye or pigment. As already mentioned before,the usual dyes and pigments can be chosen such that they prevent orreduce halation in the silver halide emulsions used, which have anydesired photosensitivity range comprised between 300 and 900 nm.

According to one embodiment the aluminium foil constitutes the solereceptor material for the silver image in that the aluminium itselftakes part actively in the formation of the silver image byelectrochemically reducing the transferred silver complexes. The use ofsuch an aluminium foil as sole receptor material has been described ini.a. EP-A 0059008.

According to a frequently used alternative embodiment the grained,anodized, and optionally sealed aluminium foil can be provided with asilver-receptive stratum comprising development nuclei that initiate thephysical development of the transferred silver complexes to form asilver image therein. Suitable development nuclei are sulphides of heavymetals e.g. sulphides of antimony, bismuth, cadmium, cobalt, lead,nickel, palladium, platinum, silver, and zinc. Especially suitabledevelopment nuclei are NiS.Ag₂ S nuclei as described in U.S. Pat. No.4,563,410. Other suitable development nuclei are salts such as e.g.selenides, polyselenides, polysulphides, mercaptans, and tin (II)halides. Heavy metals or salts thereof and fogged silver halide aresuitable as well. The complex salts of lead and zinc sulphides areactive both alone and when mixed with thioacetamide, dithiobiuret, anddithiooxamide. Heavy metals, preferably silver, gold, platinum,palladium, and mercury can be used in colloidal form.

The silver-receptive stratum may incorporate at least one antihalationdye or pigment to promote the image sharpness. Again, the usual dyes andpigments can be chosen depending upon the desired absorption spectrum ofthe silver-receptive stratum relative to the spectral sensitivity of thesilver halide emulsion layer(s) used.

The silver halide emulsion layer can be any photosensitive silver halideemulsion comprising a hydrophilic colloid binder. The photosensitivesilver halide used in the present invention may comprise silverchloride, silver bromide, silver bromoiodide, silver chlorobromoiodideand the like, or mixtures thereof. To obtain a sufficiently high rate ofdissolution of the silver halide and a satisfactory gradation necessaryfor graphic purposes a silver halide emulsion mainly comprising silverchloride is often used. This silver chloride emulsion may comprise minoramounts of silver bromide and/or silver iodide.

The silver halide emulsions may be coarse- or fine-grained and can beprepared by any of the well known procedures e.g. single jet emulsions,double jet emulsions such as Lippmann emulsions, ammoniacal emulsions,thiocyanate- or thioether-ripened emulsions such as those described inU.S. Pat. No. 2,222,264, 3,320,069, and 3,271,157. Surface imageemulsions may be used or internal image emulsions may be used such asthose described in U.S. Pat. No. 2,592,250, 3,206,313, and 3,447,927. Ifdesired, mixtures of surface and internal image emulsions may be used asdescribed in U.S. Pat. No. 2,996,382. The silver halide particles of thephotographic emulsions may have a regular crystalline form such as acubic or octahedral form or they may have a transition form.Regular-grain emulsions are described in e.g. J. Photogr. Sci., Vol. 12,No. 5, Sept./Oct. 1964, pp. 242-251. The silver halide grains may alsohave an almost spherical form or they may have a tabular form (so-calledT-grains), or may have composite crystal forms comprising a mixture ofregular and irregular crystalline forms. The silver halide grains mayhave a multilayered structure having a core and shell of differenthalide composition. Besides having a differently composed core and shellthe silver halide grains may comprise also different halide compositionsand metal dopants inbetween.

Two or more types of silver halide emulsions that have been prepareddifferently can be mixed for forming a photographic emulsion for use ina photographic material treated with a processing liquid according tothe present invention.

The average size of the silver halide grains may range from 0.2 to 1.2μm, and the size distribution can be homodisperse or heterodisperse. Ahomodisperse size distribution is obtained when 95% of the grains have asize that does not deviate more than 30% from the average grain size.

In addition to silver halide the emulsions may also comprise organicsilver salts such as e.g. silver benzotriazolate and silver behenate.

The silver halide crystals can be doped with Rh³⁺, Ir⁴⁺, Cd²⁺, Zn²⁺,Pb²⁺.

The photographic emulsions can be prepared from soluble silver salts andsoluble halides according to different methods as described e.g. by P.Glafkides in "Chimie et Physique Photographique", Paul Montel, Paris(1967), by G. F. Duffin in "Photographic Emulsion Chemistry", The FocalPress, London (1966), and by V. L. Zelikman et al in "Making and CoatingPhotographic Emulsion", The Focal Press, London (1966).

The emulsion can be desalted in the usual ways e.g. by dialysis, byflocculation and re-dispersing, or by ultrafiltration.

Besides the silver halide another essential component of aphotosensitive emulsion layer is the binder. The binder is a hydrophiliccolloid, usually a protein, preferably gelatin. Gelatin can, however, bereplaced in part or integrally by synthetic, semi-synthetic, or naturalpolymers. Synthetic substitutes for gelatin are e.g. polyvinyl alcohol,poly-N-vinyl pyrrolidone, polyvinyl imidazole,, polyvinyl pyrazole,polyacrylamide, polyacrylic acid, and derivatives thereof, in particularcopolymers thereof. Natural substitutes for gelatin are e.g. otherproteins such as zein, albumin and casein, cellulose, saccharides,starch, and alginates. In general, the semi-synthetic substitutes forgelatin are modified natural products e.g. gelatin derivatives obtainedby conversion of gelatin with alkylating or acylating agents or bygrafting of polymerizable monomers on gelatin, and cellulose derivativessuch as hydroxyalkyl cellulose, carboxymethyl cellulose, phthaloylcellulose, and cellulose sulphates.

Apart from negative-working silver halide emulsions that are preferredfor their high photosensitivity, use can be made also of direct-positivesilver halide emulsions that produce a positive silver image in theemulsion layer(s) and a negative image on the aluminium foil.

For instance, direct-positive emulsions of the type described in U.S.Pat. No. 3,062,651 may be employed. In direct-positive emulsions anon-hardening fogging agent such as stannous chloride and formamidinesulphinic acid can be used.

The emulsions can be chemically sensitized e.g. by addingsulphur-containing compounds during the chemical ripening stage e.g.allyl isothiocyanate, allyl thiourea, and sodium thiosulphate. Alsoreducing agents e.g. the tin compounds described in BE-A 493,464 and568,687, and polyamines such as diethylene triamine or derivatives ofaminoethane-sulphonic acid can be used as chemical sensitizers. Othersuitable chemical sensitizers are noble metals and noble metal compoundssuch as gold, platinum, palladium, iridium, ruthenium and rhodium. Thismethod of chemical sensitization has been described in the article of R.Koslowsky, Z. Wiss. Photogr. Photophys. Photochem. 46, 65-72 (1951).

The emulsions can also be sensitized with polyalkylene oxidederivatives, e.g. with polyethylene oxide having a molecular weight of1000 to 20,000, or with condensation products of alkylene oxides andaliphatic alcohols, glycols, cyclic dehydration products of hexitols,alkyl-substituted phenols, aliphatic carboxylic acids, aliphatic amines,aliphatic diamines and amides. The condensation products have amolecular weight of at least 700, preferably of more than 1250. It isalso possible to combine these sensitizers with each other as describedin BE-A 537,278 and GB-A 727,982.

The spectral photosensitivity of the silver halide can be adjusted byproper sensitization to any desired spectral range comprised between 300and 900 nm by means of the usual mono- or polymethine dyes such asacidic or basic cyanines, hemicyanines, oxonols, hemioxonols, styryldyes or others, also tri- or polynuclear methine dyes e.g. rhodacyaninesor neocyanines. Such spectral sensitizers have been described by e.g. F.M. Hamer in "The Cyanine Dyes and Related Compounds" (1964) IntersciencePublishers, John Wiley & Sons, New York. The spectral photosensitivityof the silver halide can also be adjusted for exposure by laser lighte.g. helium-neon laser light, argon laser light, and solid state laserlight. Dyes that can be used for adjusting the photosensitivity to laserlight have been described in i.a. JA-A 62284344, 62284345, 62141561,62103649, 62139555, 62105147, 62105148, 62075638, 62062353, 62062354,62062355, 62157027, 62157028, 62113148, 61203446, 62003250, 60061752,55070834, 51115821, 51115822, 51106422, 51106423, 51106425; DE-A3,826,700; U.S. Pat. No. 4,501,811, 4,725,532, 4,784,933; GB-A1,467,638; and EP-A 100,654 and in documents cited therein.

The silver halide can also be sensitized with a dye or a mixture of dyesproviding a spectral sensitivity mainly in the range of 400 to 500 nmand not extending the sensitivity substantially beyond 500 nm so thatthe sensitivity at 530 nm is at least 10² lower than that at 500 nm andthat the resulting photosensitive monosheet layer assemblage can behandled in yellow safe-light conditions prior to the photo-exposure,said conditions corresponding to the light transmitted by a cut-offfilter having at 500 nm a density of at least 2.5, at 530 nm a densitynot higher than 2.0, at 540 nm a density not higher than 1.0. at 550 nma density not higher than 0.4, at 560 nm a density not higher than 0.2,and beyond 580 nm a density not higher than 0.1. Suitable dyes that canbe used for that purpose have been described in e.g. U.S. Pat. No.4,686,170. Image-wise exposure of silver halide emulsions sensitized inthis way can be performed by means of lasers emitting below 500 nm e.g.an argon laser emitting at 488 nm. A particular advantage in the case ofimage-wise exposure of direct-positive silver halide emulsionssensitized with such dyes and used for the production of printing platesaccording to the DTR-process in general is that an economical and quickexposure by laser under yellow safelight conditions is possible, sinceonly the areas of the direct-positive silver halide emulsions thatcorrespond with the finally obtained printing areas on the printingplate have to be exposed and not the background areas.

The silver halide emulsions may contain the usual stabilizers e.g.homopolar or salt-like compounds of mercury with aromatic orheterocyclic rings such as mercaptotriazoles, simple mercury salts,sulphonium mercury double salts and other mercury compounds. Othersuitable stabilizers are azaindenes, preferably tetra- orpenta-azaindenes, especially those substituted with hydroxy or aminogroups. Compounds of this kind have been described by Birr in Z. Wiss.Photogr. Photophys. Photochem. 47, 2-27 (1952). Other suitablestabilizers are i.a. heterocyclic mercapto compounds e.g.phenylmercaptotetrazole, quaternary benzothiazole derivatives, andbenzotriazole.

The silver halide emulsions may comprise other ingredients e.g.antifogging agents, developers and/or development accelerators, wettingagents, and hardeners. Optionally, the silver halide emulsions maycomprise matting agents or spacing agents e.g. finely divided silicaparticles and polymer beads as described U.S. Pat. No. 4,614,708, topromote an effective vacuum suction of the photosensitive material invacuum contact exposure units.

Whereas according to known methods in the art it is indeed customary tosubstantially harden the silver halide emulsions, the binder of whichusually is gelatin, to prevent an undesired transfer of gelatin to thealuminium foil, hardening is not necessary according to the presentinvention. The transfer of gelatin to the aluminium foil indeed givesrise to the above described disadvantages. The intermediate layeractually prevents the binder of the silver halide emulsion or emulsionsfrom being transferred to the aluminium foil or substantially reducessuch transfer. The hardening degree of the silver halide emulsion layercan thus be adjusted at wish.

The silver halide emulsion may comprise light-screening dyes that absorbscattering light and thus promote the image sharpness and, as aconsequence thereof, the sharpness of the final printed copy.

Light-absorbing dyes that can be used as light-screening dyes have beendescribed in i.a. U.S. Pat. No. 4,092,168, U.S. Pat. No. 4,311,787, DE-A2,453,217, and GB-A 7,907,440. Alternatively, light-absorbing dyes canbe incorporated into a thin supplemental intermediate layer providedbetween said at least one silver halide emulsion layer and saidintermediate layer. Again, the light-screening dyes or light-absorbingdyes can be chosen depending upon the desired absorption spectrum of thelayer comprising them.

More details about the composition, preparation and coating of silverhalide emulsions can be found in e.g. Product Licensing Index, Vol. 92,December 1971, publication 9232, p. 107-109.

As an interesting variant the silver halide emulsion may consist of afirst photosensitive direct-positive or negative silver halide emulsionin which a normal latent image is formed upon image-wise exposure and asecond low-speed silver halide emulsion whose speed is so low that no oralmost no latent image is formed therein. When the low-speed silverhalide emulsion and the photosensitive silver halide emulsion are coatedto form different layers, the resulting emulsion layers are arranged insuch a way that the low-speed emulsion is remotest from the aluminiumfoil. It is also possible to coat one single layer comprising a mixtureof said photosensitive silver halide emulsion and said low-speed silverhalide emulsion.

Thanks to the combination of photosensitive and low-speed emulsions ahigher amount of silver can migrate to form the silver image on thealuminium foil. As a result, an enhanced contrast and a high resistanceagainst mechanical wear are obtained. It has indeed been establishedthat upon application of an aqueous alkaline solution to the image-wiseexposed photosensitive silver halide emulsion in the presence of adeveloping agent and a silver halide solvent a silver image is formedtherein and that the unreduced silver halide or complexes formed thereofdiffuse from the non-silver image areas to said hydrophilic grained andanodized aluminium foil and that additionally silver halide or complexesformed thereof diffuse from the low-speed emulsion through thenon-silver image areas of the photosensitive silver halide emulsion alsoto the aluminium foil, the silver image areas of the photosensitiveemulsion forming a barrier for silver halide or complexes of thelow-speed emulsion that would also tend to migrate towards the aluminiumfoil. As a result, the silver halide or complexes thereof diffusing fromboth the photosensitive emulsion and the low-speed emulsion togetherbuild up said strengthened high-contrast silver halide on the aluminiumfoil.

The low-speed silver halide emulsion is a silver chloride-containingemulsion, the speed of which is so low, that no visible image is formedtherein under the conditions of exposure and development of thephotosensitive silver halide emulsion layer. Inasmuch as the sensitivityof the silver chloride-containing emulsion must be low, no secondripening or after-ripening thereof is needed. The low-speed silverchloride-containing emulsion, which is rinsed to be free of excesssalts, preferably is a fine-grain silver chloride-containing emulsionhaving a particle size in the range of 50 to 500 nm.

The low-speed emulsion may be a pure silver chloride emulsion or anemulsion of mixed silver halides comprising silver chloride e.g. asilver chlorobromide or chlorobromoiodide emulsion. However, thelow-speed emulsion preferably is a silver chloride emulsion for thegreater part.

In case a mixture of low-speed emulsion and of imaging emulsion iscoated to form one single layer, the amount of low-speed emulsion mayvary within wide limits. Favourable results can be obtained when theratio of low-speed silver chloride-containing emulsion to image-formingemulsion, expressed in parts by weight of silver nitrate, ranges from10:1 to 1:1. The amount of low-speed emulsion to be added depends i.a.on its own nature, on the type of image-forming emulsion used, and onthe effect desired. It can be determined easily by routineers in the artby making a few comparative tests.

When separate layers of low-speed emulsion and of imaging emulsion areused, the ratio expressed in parts by weight of silver nitrate of saiddifferent layers, also ranges from 10:1 to 1:1.

An optional supplemental intermediate layer, which may be presentbetween said at least one silver halide emulsion layer and saidintermediate layer comprising the hydrophobic polymer beads according tothe present invention, may incorporate one or more ingredients such asi.a. antihalation dyes or pigment, developing agents, silver halidesolvents, base precursors, and anticorrosion substances.

With respect to the above-mentioned embodiment of the present invention,according to which a temporary base is coated first with at least onesilver halide emulsion layer and next with said intermediate layercomprising hydrophobic polymer beads, and according to which theresulting photosensitive layer packet comprising said intermediate layerand said at least one silver halide emulsion layer is transferred ontothe wet hydrophilic grained and anodized surface of an aluminium foil,the following particulars can be given.

The adherence of said at least one silver halide emulsion layer to saidtemporary base should be such that an easy stripping off from thetemporary base is possible after pressing said photosensitive layerpacket against said wet hydrophilic grained and anodized aluminium foil.Therefore, a relatively hydrophobic temporary base is used, whichpreferably is flexible and is made of i.a. cellulose triacetate,polystyrene, polyester e.g. polyethylene terephthalate, and copoly(vinylacetate/vinyl chloride). Preferably, the temporary base is an unsubbedcellulose triacetate or polyethylene terephthalate film base. Thethickness of the cellulose triacetate or polyethylene terephthalate filmbase may vary between wide limits, but preferably is approximately 100μm.

The said at least one silver halide emulsion layer can be composed insuch a way that its adherence to the temporary base in wet state is lessthan that in dry state. Optionally, hygroscopic substances e.g.water-soluble organic hygroscopic compounds such as glycerol, or wettingand/or plasticizing agents can be added to said at least one silverhalide emulsion layer to adjust its adherence.

Other temporary bases having a repelling power for wet gelatin coatingsare e.g. a paper base coated with a polyethylene layer, a paper baseimpregnated with wax, a paper base coated with a layer of cellulosenitrate, a paper base coated with a layer of insolubilized polyvinylalcohol, and a layer of alginic acid insolubilized with an alkalineearth metal salt. Temporary bases comprising a paper support should beremoved before the photo-exposure, whereas transparent film bases mayremain during the photo-exposure and are in that case removedafterwards.

The transfer of the transferable layers onto the aluminium foil can becarried out in an apparatus, in which the aluminium foil is moistenedand the wet aluminium foil and the photosensitive layer packet arepressed together between rollers. An apparatus particularly suitable foruse in transferring transferable layers from the temporary base to thewet aluminium foil has been described in U.S. Pat. No. 4,701,401.Suitable apparatus for that purpose are the AGFAPROOF TR unit and theAGFAPROOF TR S unit, both marketed by AGFA-GEVAERT, Belgium.

A wide choice of cameras for exposing the photosensitive silver halideemulsion exists on the market. Horizontal, vertical and darkroom typecameras and contact-exposure apparatus are available to suit anyparticular class of reprographic work. The photosensitive silver halideemulsion(s) used in the layer assemblages according to the presentinvention can also be exposed with the aid of i.a. laser recorders andcathode rays tubes.

The development and diffusion transfer are effected with the aid of anaqueous alkaline solution in the presence of at least one developingagent and at least one silver halide solvent. The developing agent(s)and/or the silver halide solvent(s) can be incorporated in the aqueousalkaline solution and/or in said at least one silver halide emulsionlayer and/or in said intermediate layer and/or in a supplementalhydrophilic colloid layer in water-permeable relationship with said atleast one silver halide emulsion layer. The latter supplementalhydrophilic colloid layer can be coated on top of said at least onesilver halide emulsion layer remotest from said aluminium foil e.g. itcan be provided between said temporary base and said at least one silverhalide emulsion layer.

The silver halide solvent can also be incorporated at least in part inthe silver-receptive stratum coated on the aluminium foil. When theaqueous alkaline solution does not comprise the developing agent(s), itis merely an activating liquid that is capable of dissolving thedeveloping agent(s) contained in one of the layers.

The conventional developing agents for DTR-processing are ahydroquinone-type compound in combination with a secondary developingagent of the class of 1-phenyl-3-pyrazolidinone compounds andp-N-methylminophenol. Particularly useful 1-phenyl-3-pyrazolidinonedeveloping agents are 1-phenyl-3-pyrazolidinone,1-phenyl-4-methyl-3-pyrazolidinone,1-phenyl-4-ethyl-5-methyl-3-pyrazolidinone, and1-phenyl-4,4-dimethyl-3-pyrazolidinone.

The hydroquinone-type compound is e.g. hydroquinone,methyl-hydroquinone, or chlorohydroquinone.

The silver halide solvent, which acts as a complexing agent for silverhalide, preferably is a water-soluble thiosulphate or thiocyanate e.g.sodium, potassium, or ammonium thiosulphate and sodium, potassium, orammonium thiocyanate.

Other suitable silver halide solvents are i.a. sulphites, amines, andsuch alkanolamines like e.g. ethanolamine, diethanolamine,triethanolamine, diisopropanolamine, 2-methyl -aminoethanol, 2-ethyl-aminoethanol, 2-dimethylaminoethanol, 2-diethyl-aminoethanol,2-methyl-2-amino-1-propanol, 1-diethylamino-2-propanol,3-diethylamino-1-propanol, isopropylaminoethanol, 3-amino-1-propanol,2-methyl-2-amino-1,3-propanediol, benzyldiethanolamine, and2-(hydroxymethyl )-2-amino- 1,3-propanediol.

Further suitable silver halide solvents are those disclosed in "TheTheory of the Photographic Process" 4th Ed., edited by T. H. James,pages 474-475. Further interesting silver halide solvents have beendescribed in i.a. U.S. Pat. No. 2,857,276 and 4,297,430. Among these arecyclic imide compounds such as e.g. uracil and 5,5-dialkylhydantoins.Other suitable silver halide solvents are the alkyl sulfones.

Combinations of different silver halide solvents can be used and it iseven possible to incorporate at least one silver halide solvent into asuitable layer and add at least one other silver halide solvent to thedeveloping solution.

The following quantitative ranges given for the developing agents,silver halide solvents, and sulphite apply to the amount of thesecompounds present as solutes in the aqueous alkaline solution during theDTR-processing, whether these compounds make part of the aqueousalkaline solution, which in that particular case actually is an aqueousalkaline developing solution, or were dissolved from the layerscontaining them upon application thereto of the aqueous alkalinesolution, in which case it is an activating solution.

A suitable quantitative combination of hydroquinone and at least onesecondary developing agent of the class of 1-phenyl-3-pyrazolidinonesand p-N-methyl-aminophenol comprises hydroquinone in an amount not lowerthan 0.078 mole per litre of aqueous alkaline solution and the secondarydeveloping agent(s) in an amount not lower than 0.0080 mole per litre,the molar ratio of hydroquinone to said secondary developing agent(s)not being lower than 9.7. Preferred amounts of hydroquinone are in therange of 0.05 mole to 0.25 mole per litre and preferred amounts ofsecondary developing agent(s) in the range of 1.8×10⁻³ to 2.0×10⁻² moleper litre.

The aqueous alkaline solution may comprise sulphite e.g. sodium sulphitein an amount ranging from 40 g to 180 g per litre, preferably from 60 to160 g per litre, and thiosulphate and/or thiocyanate in an amountranging from 5 g to 20 g per litre.

The pH of the aqueous alkaline solution preferably is at least 12, butdepends on the type of silver halide emulsion material to be developed,intended development time, and processing temperature.

The processing conditions such as temperature and time may vary withinbroad ranges provided the mechanical strength of the materials to beprocessed is not adversely influenced and no decomposition takes place.

The aqueous alkaline solution may comprise such alkali-providingsubstances like hydroxides of sodium and potassium, alkali metal saltsof phosphoric acid and/or silicic acid e.g. trisodium phosphate,orthosilicates, metasilicates, hydrodisilicates of sodium or potassium,and sodium carbonate. The alkali-providing substances can be substitutedin part or wholly by alkanolamines.

The aqueous alkaline solution may comprise at least one alkanolamine toimprove its life-time and performance for the DTR-process. Suitablealkanolamines are i.a. N,N,N-triethanolamine,2-amino-2-hydroxymethyl-propan-1,3-diol, N-methyl-diethanolamine,N-ethyl-diethanolamine, diisopropanolamine, N,N-diethanolamine,3,3'-amino-dipropanol, 2-amino-2-methyl-propan-1,3-diol,N-propyl-diethanolamine, N-butyl-diethanolamine, N,N-dimethyl-ethanolamine, N,N-diethyl-ethanolamine,N,N-diethyl-isopropanolamine, 1-amino-propan-2-ol, N-ethanolamine,N-methyl-ethanolamine, N-ethyl-ethanolamine, 3-amino-propanol,4-amino-butanol, and 5-amino-pentan-1-ol.

According to a preferred embodiment described in Research Disclosure27939 (July 1987) pages 450-451 the aqeuous alkaline solution comprisesat least one tertiary alkanolamine having a pKa value higher than 8.5.More preferably, the solution comprises two or more tertiaryalkanolamines having a pKa value higher than 9.0.

The aqueous alkaline solution may further comprise silver-imagehydrophobizing compounds e.g. heterocyclic mercapto compounds. Theaddition of heterocyclic mercapto compounds more particularly amercapto-1,3,4-thiadiazole to a developing liquid for the purpose ofhydrophobizing the silver image formed according to the DTR-process onan aluminium foil has been described already in DE-A 1,228,927. Othersuitable mercapto-thiadiazoles that can be added to the aqueous alkalinesolution have been disclosed in U.S. Pat. No. 4,563,410. Anothersuitable hydrophobizing compound is 2-mercapto-5-heptyl-oxa-3,4-diazole.

These hydrophobizing compounds can be added to the aqueous alkalinesolution in an amount of preferably 0.1 to 3 g per litre and preferablyin admixture with 1-phenyl-5-mercaptotetrazole, the latter compoundbeing used in amounts of e.g. 50 mg to 1.2 g per litre of solution,which may contain a minor amount of ethanol to improve the dissolutionof said compounds.

The aqueous alkaline solution may comprise other ingredients such ase.g. oxidation preservatives, a compound releasing bromide ions,calcium-sequestering compounds, anti-sludge agents, and hardenersincluding latent hardeners.

Regeneration of the aqueous alkaline solution according to known methodsis, of course, possible, whether the solution incorporates developingagent(s) and/or silver halide solvent(s) or not.

The development may be stopped--though this is often not necessary--witha so-called stabilization liquid, which actually is an acidic stop-bathhaving a pH preferably in the range of 5 to 6.

Buffered stop bath compositions comprising a mixture of sodiumdihydrogen orthophosphate and disodium hydrogen orthophosphate andhaving a pH in said range are preferred.

The development and diffusion transfer can be initiated in differentways e.g. by rubbing with a roller, by wiping with an absorbent meanse.g. with a plug of cotton or sponge, or by dipping the material to betreated in the liquid composition. Preferably, they proceed in anautomatically operated apparatus such as the CR 430, CR 740, or CR1100-Processors marketed by AGFA-GEVAERT, Belgium. They are normallycarried out at a temperature in the range of 18° C. to 30° C.

After formation of the silver image on the aluminium foil the excess ofalkaline solution still present on the foil is eliminated, preferably byguiding the foil through a pair of squeezing rollers.

According to a preferred embodiment of uncovering the imaged aluminiumfoil the intermediate layer and the emulsion layer(s) wet with alkalinesolution or moistened with unheated water or unheated aqueous mediumapplied subsequent to the removal from said alkaline solution areseparated from the imaged aluminium foil by an airstream directed on anedge of said monosheet layer assemblage.

According to another embodiment of uncovering the imaged aluminium foilthe developed monosheet layer assemblage is rinsed with unheated wateror an unheated aqueous medium so that the intermediate layer and theemulsion layer(s) are removed from the imaged aluminium foil.

According to another embodiment of uncovering the imaged aluminium foilthe intermediate layer and the emulsion layer(s) can be removed also byslightly agitating the developed monosheet layer assemblage while beingdipped in unheated water or unheated aqueous medium or by slightlyagitating a tray comprising unheated water or unheated aqueous medium inwhich said developed monosheet layer assemblage has been immersed.According to a convenient method of rinsing away said layers, thedeveloped monosheet layer assemblage is held under a spray or jet ofunheated water or unheated aqueous medium. The mechanical pressure of awater jet or spray directed onto these layers suffices to detach themfrom the aluminium foil. The unheated aqueous medium used to detach theintermediate layer and the emulsion layer(s) by rinsing may compriseingredients such as i.a. weak acidifying agents, wetting agents, andhardeners including latent hardeners.

According to a further embodiment of uncovering the imaged aluminiumfoil the monosheet layer assemblage is pressed with its side showing theemulsion layer(s) while being moistened with unheated water or withunheated aqueous medium against a receiving sheet such as e.g. a paperor film base coated with a hardened gelatin layer comprising a mattingagent or against a porous web as described in Research Disclosure 23017(June 1983) pages 223-4 and said imaged aluminium foil is peeled offfrom the emulsion layer(s) and the intermediate layer, which aresupported by the receiving sheet and strongly adhere thereto. Accordingto an alternative embodiment the emulsion layer(s) and the intermediatelayer can also be removed by scraping off or by wiping off e.g. with asponge.

According to an alternative method of separating said at least onesilver halide emulsion layer and said intermediate layer from the imagedhydrophilic grained and anodized surface, the monosheet layer assemblageis placed with its side showing said at least one silver halide emulsionlayer during the period of time that starts with the application of saidaqueous alkaline solution and ends with said formation of a silver imageon said hydrophilic grained and anodized surface in contact with areceiving means, said at least one silver halide emulsion layer and saidintermediate layer after said formation of a silver image while stillbeing wet with said aqueous alkaline solution, having an adherence tosaid receiving means that is stronger than that to the imagedhydrophilic grained and anodized surface. Next, said at least one silverhalide emulsion layer and said intermediate layer adhering to andsupported by said receiving means are peeled off from the imagedhydrophilic grained and anodized surface. According to this method theemulsion layer(s) and intermediate layer are separated from the imagedhydrophilic grained and anodized surface after the silver image has beenformed thereon and while they are still wet with said aqueous alkalinesolution. For carrying out this process the photo-exposed monosheetlayer assemblage and a receiving means e.g. a paper or film base coatedwith a hardened gelatin layer comprising a matting agent can beintroduced in a processing device through different inlet openings andafter completion of the development and of the image transfer pressedtogether in the nip of two rollers. When the contacting monosheet layerassemblage and the receiving means leave the nip between the tworollers, a suitable and appropriately mounted separating blade canseparate the imaged aluminium foil from the receiving means, to whichsaid intermediate layer and said emulsion layer(s) remain adhering. As aconsequence the thus obtained bared aluminium foil carrying thetransferred silver image does not need any further rinsing, althoughrinsing with water is possible, of course, if desired.

It is common practice in the art to subject the imaged surface of thealuminium foil to a chemical treatment that increases the hydrophilicityof the non-silver image parts and the oleophilicity of the silver image.

This chemical after-treatment is preferably carried out with alithographic composition often called fixer, which comprises at leastone compound enhancing the ink-receptivity and/or lacquer-receptivity ofthe silver image, and also comprises at least one compound that improvesthe ink-repelling characteristics of the aluminium. In U.S. Pat. No.4,563,410 an interesting method for hydrophobizing the silver image hasbeen described.

Suitable ingredients for the fixer are e.g. organic compounds containinga mercapto group such as dodecylmercaptans, benzothiazole-2-thiol,1,3,4-thiadiazole-2-thiol, 1-phenyl-1-H-tetrazole-5-thiol,triazinethiols e.g. 1-octyl-1,2,4,5-tetrahydro-S-triazine-5-thiol, andcompounds containing a thioasid or a thioamide group. Additivesimproving the oleophilic ink-resiliency of the bare anodized aluminiumfoil areas are e.g. carbohydrates such as acid polysaccharides like gumarabic, carboxymethylcellulose, sodium alginate, propylene glycol esterof alginic acid, hydroxyethyl starch, dextrin, hydroxyethylcellulose,polyvinyl pyrrolicone, polystyrene sulphonic acid, and polyvinylalcohol. Optionally, hygroscopic substances e.g. sorbitol, glycerol,tri(hydroxyethyl)ester of glycerol, and turkey red oil may be added.Furthermore, phosphoric acid and cationic surface-active compounds suchas hexadecyl trimethyl ammonium bromide can also be added to the fixer.A suitable fixer is e.g. a composition comprising a solution of1-octyl-l,2,4,5-tetrahydro-S-triazine-5-thiol in acetone or a suspensionthereof in a solution of gum arabic. Other suitable fixers have beendescribed in i.a. U.S. Pat. No. 4,062,682 and U.S. Pat. No. 4,563,410.

At the moment the treatment with the fixer is started the surfacecarrying the silver pattern may be in dry or wet state. In general, thetreatment with the fixer does not take long, usually not longer thanabout 30 seconds and it may be carried out immediately after theprocessing and uncovering steps.

The fixer can be applied in different ways such as by rubbing with aroller, by wiping with an absorbent means e.g. with a plug of cotton orsponge, or by dipping the material to be treated in the fixer. Theimage-hydrophobizing step of the printing plate may also proceedautomatically by conducting the printing plate through a device having anarrow channel filled with the fixer and conveying the printing plate atthe end of the channel between two squeezing rollers removing the excessof liquid. Suitable devices for automatic treatment with a fixer are theCRF 45 and CRF 85 fixing units, both marketed by AGFA-GEVAERT, Belgium.

As soon as the aluminium foil carrying the silver image has been treatedwith the fixer, it is ready to be inked and used as a printing plate sothat a treatment thereof with a lacquer composition for strengtheningthe printing areas is not necessary. The printing plate has to be wet atthe stage the greasy printing ink is applied. This is a generally knownfact in the art and it is usual to apply an aqueous liquid before theprinting ink is applied. This can be done by means of a wet sponge or bymeans of the fountain arrangements (dampening system) of the printingmachine.

For the production of long-run printing plates requiring more than100,000 prints a second after-treatment consisting in applying a lacqueronto the silver image areas may be advisable. For this purpose, lacquersbased on phenol- or cresol-formaldehyde alkyd resins and/or epoxyresinsare commonly used.

Another useful resin for such a lacquer is a mixture of homopolymers andcopolymers of styrene, ortho-, meta-, or para-vinyltoluene and indeneunits. Cyclohexanone can be used as solvent and linseed oil asplasticizer. Examples of suitable lacquer compositions have beendescribed in i.a. GB-A 968,706 and 1,071,163 and in CA-A 686,284.

A lithographic composition in which the fixer and lacquer are combinedto one composition has been described in e.g. GB-A 969,072.

The following examples illustrate the present invention.

EXAMPLE 1

Three identical photosensitive monosheet layer assemblages were made asfollows.

An aluminium foil having a thickness of 0.20 mm was grained, anodized,and sealed according to the method described in Example 1 of U.S. Pat.No. 3,861,917. The centre line average value obtained by the grainingwas 0.5 μm. The anodization layer having a weight of 2.7 g per m2 wascoated with a silver-receptive stratum from a silver sol in watercomprising no binder, prepared according to the Carey Lea method, theresulting stratum having a weight in dried condition of 4 mg of silverper m2.

The silver-receptive stratum was covered with a substantially unhardenedphotosensitive negative-working cadmium-free gelatin silverchlorobromoiodide emulsion layer (97.98/2/0.02 mol %), the silver halidebeing coated in an amount corresponding to 2.40 g of silver nitrate perm2 and the gelatin content of the resulting photosensitive emulsionlayer being 1.58 g/m2.

The resulting 3 identical photosensitive monosheet layer assemblages arecalled "Comparison A", "Comparison B", and "Comparison C" respectivelyhereinafter.

A photosensitive monosheet layer assemblage called "Invention" was thenmade in the same way as described for Comparison A, B, and C, with theonly difference that an intermediate layer comprising hydrophobicpolymer beads was provided between the silver-receptive stratum and thephotosensitive emulsion layer. The intermediate layer was coated on thedry silver-receptive stratum from a composition comprising 50 ml of a20% dispersion of polymethyl methacrylate beads in a mixture of equalvolumes of water and ethanol, which beads have been prepared asdescribed in Preparation example 1 hereinbefore and have an averagediameter of 1.0 μm, 2.5 g of Helioechtpapierrot BL (trade mark for a dyesold by BAYER AG, D-5090 Leverkusen, West-Germany), 2.5 g of saponine,1.25 g of sodium oleylmethyltauride, and 300 ml of demineralized waterin such a way that the resulting dried layer comprised 0.5 g ofpolymethyl methacrylate beads per m2.

The 4 photosensitive monosheet layer assemblages were exposedidentically through a contact screen in a process-camera and immersedfor 8 s at 25° C. in a freshly made developing solution having thefollowing ingredients in a CR 430 processor marketed by AGFA-GEVAERT,Belgium:

    ______________________________________                                        carboxymethylcellulose     18     g                                           sodium hydroxide           22.5   g                                           anhydrous sodium sulphite  120    g                                           hydroquinone               20     g                                           1-phenyl-3-pyrazolidinone  3      g                                           potassium bromide          0.75   g                                           anhydrous sodium thiosulphate                                                                            7.5    g                                           ethylene diamine tetraacetic acid tetrasodium salt                                                       2      g                                           demineralized water to make                                                                              1000   ml                                          pH (25° C.) = 13                                                       ______________________________________                                    

The initiated diffusion transfer was allowed to continue for 30 s toform a silver image on the aluminium foil.

To remove the developed silver halide emulsion layer and theintermediate layer (Invention) from the imaged aluminium foil, each ofthe 4 developed monosheet layer assemblages was rinsed for 30 s with awater jet. In case the emulsion layer did not detach with unheated waterthe test was repeated with a fresh identical layer assemblage and thetemperature of the rinsing water was enhanced. The temperature used foreach of the 4 monosheet layer assemblages is specified in Table 1hereinafter.

Next, the imaged surface of the aluminium foil was rubbed with one ofthe fixers A or B as specified in Table 1 hereinafter to enhance thewater-receptivity of the non-image areas and to make the image areasoteophilic ink-receptive. Fixer A had the following composition:

    ______________________________________                                        10% aqueous n-hexadecyl trimethyl ammonium                                                               25     ml                                          chloride                                                                      20% aqueous solution of polystyrene sulphonic acid                                                       100    ml                                          potassium nitrate          12.5   g                                           citric acid                20.0   g                                           1-phenyl-5-mercaptotetrazole                                                                             2.0    g                                           sodium hydroxide           5.5    g                                           water to make              1000   ml                                          pH (20° C.) = 4                                                        ______________________________________                                    

The composition of Fixer B was identical to that of Fixer A with theonly difference that Fixer B additionally comprised 20 g of trypsin perlitre.

Each of the printing plates obtained was placed on an Heidelberg offsetprinting press, type GTO, marketed by HEIDELBERGER DRUCKMASCHINEN AG,D-6900 Heidelberg, West-Germany.

Each printing plate was inked with a commercially available KAST+EHINGER123W ink and then used for printing copy sheets of paper.

The lithographic quality of each 25th print was evaluated as for itslithographic quality. The results of this evaluation are given in thefollowing Table 1. Four levels of appreciation of the lithographicquality were attributable. The term "perfect" was used when the imageprinted on the paper was an exact reproduction of the silver image onthe printing plate, had a perfect density, and showed no deficienciessuch as pin-holes. The term "good" could be used when the image was analmost exact reproduction of the silver image on the printing plate, hadan acceptable density, and showed only few deficiencies. The term "bad"was used when the printed image was a recognizable reproduction, but hadan unpleasantly weak density, and showed many deficiencies. The term"very bad" was used when the printed image was unrecognizable anduseless.

                  TABLE 1                                                         ______________________________________                                                 Temperature of       Lithographic                                             rinsing water                                                                           Fixer      quality                                         ______________________________________                                        Comparison A                                                                             20° C.                                                                             A          very bad                                    Comparison B                                                                             50° C.                                                                             A          bad                                         Comparison C                                                                             50° C.                                                                             B (trypsin)                                                                              perfect                                     Invention  20° C.                                                                             A          perfect                                     ______________________________________                                    

With the "Invention" printing plate a run of 100,000 prints on paper wascarried out. The quality of the last print was still perfect. Noabnormal wear or decline in image sharpness was visible.

EXAMPLE 2

A series of photosensitive monosheet layer assemblages comprising anintermediate layer comprising hydrophobic polymethyl methacrylate beadsprepared as described in Preparation example 1 hereinbefore and havingan average diameter of 1.0 μm were made as described in Example 1, theonly differentiation between the photosensitive monosheet layerassemblages being that the weight of polymethyl methacrylate beadspresent per m2 of dried intermediate layer was different as indicated inTable 2 hereinafter.

The intermediate layer was covered with a photosensitive silverchlorobromoiodide emulsion layer as described in Example 1.

The procedure of exposure, DTR-development, treatment with fixer A, andprinting described in Example 1 was repeated for each of the layerassemblages.

The lithographic quality of each 25th print was evaluated as describedin Example 1. The coating weight of the intermediate layer and theresults of this evaluation are given in Table 2. Furthermore, thesharpness of the silver image on the imaged aluminium foil and of each25th print obtained therefrom is measured with the aid of the FOGRAPrecision Measuring Strip PMS I as described in Fogra Praxis Report No34 published by Deutsche Forschungsgesellschaft fur Druck- undReproduktionstechnik, P.O. 800469, 8000 Munich 80 - W. Germany.

Table 2 also compares the yield of silver (expressed in silver nitrate).By yield of silver is meant the percent ratio by weight of transferredsilver present on the aluminium foil versus the silver of thetransferable silver ucomplexes present in the emulsion layer. In thecolumn entitled "wash-off" an appreciation is given for the removabilityof the intermediate layer and the silver halide emulsion layer with ajet of unheated water.

                  TABLE 2                                                         ______________________________________                                        Coating weight                                                                          Sharpness  Yield of wash- Lithographic                              of beads (g/m2)                                                                         Printing plate                                                                           silver   off   quality                                   ______________________________________                                        0.0        6 μm   55%      very  very bad                                                                bad                                             0.2       6-8 μm  59%      good  good                                      0.5        8 μm   51%      good  perfect                                   1.0        8 μm   50%      perfect                                                                             perfect                                   ______________________________________                                    

EXAMPLE 3

A photosensitive monosheet layer assemblage was made as follows.

An aluminium foil coated with a silver-receptive stratum, both asdescribed in Example 1, was covered in the given sequence at the side ofthe stratum with an intermediate layer and an emulsion layer asdescribed in Example 1.

The intermediate layer was coated on the dry silver-receptive stratumfrom a composition comprising 50 ml of a 20% dispersion of polymethylmethacrylate beads in a mixture of equal volumes of water and ethanol,which beads have been prepared as described in Preparation example 1hereinbefore and have an average diameter of 1.0 μm, 50 ml of a 5%aqueous solution of polyvinyl alcohol having a molecular weight of10,000 and comprising 95 mol % of vinyl alcohol units and 5 mol % ofvinyl acetate units, 2.5 g of Helioechtpapierrot BL (trade mark for adye sold by BAYER AG, D-5090 Leverkusen, West-Germany), 2.5 g ofsaponine, 1.25 g of sodium oleylmethyltauride, and 300 ml ofdemineralized water in such a way that the resulting dried layercomprised 0.5 g of polymethyl methacrylate beads and 0.125 g ofpolyvinyl alcohol per m2.

The procedure of exposure and DTR-development described in Example 1 wasrepeated. The developed silver halide emulsion layer and theintermediate layer while still being wet with aqueous alkalinedeveloping solution were removed from the imaged aluminium foil with anairstream as described in Example 1. Next, the imaged surface of thealuminium foil was rinsed with water and rubbed with fixer A describedin Example 1. Printing with the printing plate obtained was thencarrried out as described in Example 1 and the 1 ithographic quality ofthe 25th print was evaluated also as described in Example 1. The resultsare listed in Table 3.

                  TABLE 3                                                         ______________________________________                                        average diameter                                                                          Yield of             Lithographic                                 of beads    silver    wash-off   quality                                      ______________________________________                                        1.0         51%       very good  perfect                                      ______________________________________                                    

We claim:
 1. Photosensitive layer packet for making a lithographicaluminium printing plate according to the DTR-process, carrying on atemporary base a separable layer sandwich comprising in the givensequence at least one silver halide emulsion layer and an intermediatelayer consisting essentially of hydrophobic polymer beads prepared bypolymerization of at least one ethylenically unsaturated monomer andhaving an average diameter not lower than 0.2 μm said layer sandwichbeing transferable onto a wet separate hydrophilic grained and anodizedaluminium foil when pressed thereon.
 2. A photosensitive layer packetaccording to claim 1, wherein said temporary base is an unsubbedcellulose triacetate or polyethylene terephthalate film base.
 3. Aphotosensitive layer packet according to claim 1, wherein saidintermediate layer in dry condition comprises said hydrophobic polymerbeads in an amount of up to 80% of its total weight.
 4. A photosensitivelayer packet according to claim 1, wherein said intermediate layer hasbeen coated at a ratio of from about 0.1 to about 5 g/m2.
 5. Aphotosensitive layer packet according to claim 1, wherein saidintermediate layer comprises up to 20% by weight of any non-proteinichydrophilic film-forming polymer.